Cross-linked gels are materials which exhibit solid-like behavior resulting from continuous, three-dimensional network extending throughout a liquid. This network consists of molecules interconnected through multifunctional junctions. These junctions can be formed through covalent crosslinking, crystallization, ionic interactions, hydrogen bonding, chain entanglements, and the like.
One such cross-linked gel is resorcinol-formaldehyde gel. The resorcinol-formaldehyde cross-linked gel has numerous applications as ion exchange resins, water-based resin coatings, solvent based coating resins, resin coatings, etc. Further, the resorcinol-formaldehyde gels as carbonized products including carbon gels and activated carbon gels have their use as super capacitors, gas sensors, catalyst supports, chromatographic packing, thermal insulators, carbon black particles for reinforcing rubbers or plastics, carbon fibers for spacecraft shields, adsorbents, electrodes, Cherenkov detectors, casting molds, energy absorbers, hypervelocity particle traps, dielectrics for filters, spacers for vacuum electrodes, vacuum display spacers, pipes, and the like.
Conventionally, the resorcinol-formaldehyde cross-linked gel is prepared by a chemical process. U.S. Pat. No. 4,873,218 ('218 patent) discloses a method for preparation of resorcinol-formaldehyde cross-linked gels which includes polycondensation of resorcinol with formaldehyde under alkaline conditions. This results in formation of surface functionalized polymer “clusters”. Subsequent covalent crosslinking of these “clusters” produces gels which when processed under super critical conditions; produce low density, organic aerogels.
However, the chemical process of preparation of the resorcinol-formaldehyde cross-linked gels described in '218 patent requires precise control of various parameters, including composition of the chemicals, for example, resorcinol, formaldehyde and water, type and concentration of a catalyst, pH of the reacting mixture solution and surrounding temperature. Small variation in any of these abovementioned parameters completely changes characteristics of the final resorcinol-formaldehyde cross-linked gel products. This makes the structural and physical properties of the corresponding resorcinol-formaldehyde cross-linked gels difficult to control, non-friendly for environment, full of residues of additives and not always reproducible as well.
Further, the resorcinol-formaldehyde cross-linked gel synthesized with the chemical process of the prior art usually undergoes substantial shrinkage of, distortion of and cracking of their pore structures upon drying at ambient conditions. Also, pore structures of the resorcinol-formaldehyde cross-linked gel synthesized by the chemical process is difficult to sustain unless by drying them at supercritical conditions which, in tum, requires complicated, expensive and lengthy, tedious, procedures which cannot be applied easily on commercial scales.
A need exists therefore for providing cross-linked gels having sustainable structural properties, enhanced thermal stability, low cost, clean, easy synthesis on mass production, controllability of pore and particle structures, which overcomes the drawbacks of the prior art.